Bisphenol-A (4,4′-dihydroxy-2,2-diphenylpropane or BPA) is produced by condensation of acetone with an excess of phenol in the presence of an acidic catalyst or a cation-exchange resin. The crude product, in addition to the desired bisphenol-A and unreacted phenol, contains unwanted by-products, such as bisphenol-A isomers, trisphenols and other higher molecular weight materials. The bisphenol-A is normally separated from the crude product by a single or a series of crystallization steps, leaving a mother liquor stream enriched in unwanted by-products, a portion of which stream is removed to purge unwanted by-products from the process. Alternately, the bisphenol-A may be separated from the crude product by a single or series of distillation steps, which also creates a stream enriched in unwanted by-products, a portion of which is removed. The removed stream may contain unreacted phenol and bisphenol-A as well as the unwanted by-products. Phenol is typically recovered from the removed stream by distillation, normally vacuum distillation, leaving a residue stream concentrated in unwanted heavies which is purged from the BPA manufacturing process.
There is substantial prior art describing methods for recovering phenol and isopropenyl phenol from such residue streams to improve the economic performance of the overall BPA manufacturing process. One such method involves addition of catalytic amounts of base at elevated temperature under vacuum to decompose BPA, BPA isomers, trisphenols and other by-products into phenol and isopropenyl phenol followed by addition of catalytic amounts of acid at elevated temperature under vacuum to recover phenol (see, U.S. Pat. No. 6,191,316).
In addition, there is substantial prior art describing methods for recovering phenol and acetone from BPA and BPA residue streams. One such method involves hydrolysis of BPA residues purged from a BPA manufacturing process in the presence of water at supercritical or near-supercritical temperatures and pressures (see, “Phenol Recovery by BPA Tar Hydrolysis in Supercritical Water”, Adschiri T., Shibata R., Arai, K., Sekiyu Gakkasishi, Vol 40, No. 4, 1997, p. 291-297).
Moreover, hydrolysis of BPA and BPA residues has been shown to occur at subcritical temperatures and pressures in the presence of an aqueous solution of ammonia, alkali-metal and alkaline earth metal hydroxides and carbonates to produce phenol and acetone which can then be recovered (see, U.S. Pat. No. 3,075,015). In this process, the hydrolysis is conducted at a temperature of 150° C. to 320° C., such as 200° C. to 300° C., a pressure of 5 to 150 atmospheres and, in each of the Examples, at a molar ratio of hydroxyl to hydroxyphenyl groups of 1:1. The concentrated heavies are reacted with sodium hydroxide solution or other basic solution to convert the p,p-BPA and other compounds back to phenol and acetone. The acetone is recovered in a distillation column and the phenol is recovered by neutralization followed by steam distillation. Phenol and acetone yields using hydrolysis are substantially improved compared to methods using catalytic decomposition in the absence of water, but the caustic usage is high.
According to the present invention it has now been found that BPA-containing streams can be effectively hydrolyzed back to phenol and acetone in the presence of a basic hydroxide at molar ratios of hydroxyl to hydroxyphenyl groups significantly less than 1:1 and with relatively short residence times. In addition, it is found that acetone and phenol recovery rates increase as the ratio of water to concentrated heavies in feed stream increases. In this way, the efficiency of the process can be maximized while the caustic usage and hence cost of the process is reduced.